What’s more, a total solar eclipse is a rare event, cosmically speaking. There are close to 200 confirmed moons orbiting six major planets in our solar system (Mercury and Venus lack moons). But in only one instance is there a moon that’s the right size, and at the right distance from its planet, to just barely cover the brilliant solar disk and reveal the Sun’s wispy corona. And that’s our Moon. (Looked at another way, total solar eclipses aren’t rare; they occur roughly once every year or two somewhere on Earth. But any given spot on our planet’s surface gets darkened by the Moon’s shadow on average only once about every 400 years, so in that sense totality is indeed rare.)
The Sun’s diameter is about 400 times that of the Moon. The Sun is also (on average) about 400 times farther away. As a result, the two bodies appear almost exactly the same angular size in the sky — about ½°, roughly half the width of your pinky finger seen at arm’s length. This truly remarkable coincidence is what gives us total solar eclipses. If the Moon were slightly smaller or orbited a little farther away from Earth, it would never completely cover the solar disk. If the Moon were a little larger or orbited a bit closer to Earth, it would block much of the solar corona during totality, and eclipses wouldn’t be nearly as spectacular.
Of course the Moon doesn’t totally eclipse the Sun every month — if it did, seeing totality wouldn’t be as much of a thrill. And even when the lunar disk encroaches on the Sun, it doesn’t always completely cover the solar disk. In fact, at new Moon — the only lunar phase when a solar eclipse can occur — the Moon usually misses the Sun altogether. Given all the variables, it’s almost surprising that we see eclipses at all.
Nothing Lasts Forever
The cosmic coincidence that gives us total solar eclipses isn’t permanent. The Moon is ever so slowly moving away from our planet at rate of about 1.5 inches (3.8 centimeters) per year. As it recedes, its average apparent diameter shrinks. Eventually, the Moon will never be large enough to completely cover the Sun, and total eclipses will no longer be visible from Earth’s surface.
And when might this sad prospect come to pass? The calculation is not precise — there are many unknowns such as whether the lunar retreat will continue at a constant rate and whether the solar diameter will remain stable over a long period of time. Still, about a billion years from now, give or take a few hundred million years, the surface of Earth will experience its final total eclipse of the Sun. Annular eclipses will continue to occur, though the percentage of the solar surface hidden by the Moon will gradually decrease.
So, when you stand in the lunar shadow watching the Moon pass between Earth and the Sun, revel in the knowledge that you are witnessing one of the most unusual and spectacula…